The invention is in the field of sputtering methods for the deposition of metal coatings.
The deposition of aluminum films by vacuum vapor deposition is well-known. One use of such films is as a mirror or infrared reflector. Previously films having the highest reflectance have been prepared by vacuum evaporation of pure aluminum at high rates and at low absolute pressures which minimize the contamination of the films by oxygen and other readily absorbable gases. The preparation and properties of such films are discussed by Lewis F. Drummeter, Jr. and Georg Hass on pp. 333-339 of a chapter "Solar Absorptance and Thermal Emittance of Evaporated Coatings" which appears in "Physics of Thin Films", Georg Hass and Rudolph E. Thun, editors, Vol. 2, Academic Press, New York (1964).
A second use of aluminum layers in the metalization of intergrated circuits and other solid state electronic components. Such use is discussed by Arthur J. Learn, in "Evolution and Current Status of Aluminum Metalization", J. Electrochem. Soc., Sol. State Sci. and Tech., pp. 894-906, June (1976). Many component manufacturers use automatic mask-aligning equipment which requires metal layers having high specular reflectance. Other manufacturers use positive photoresist techniques for which non-specular reflecting films are preferred. In addition to layers of essentially pure aluminum, many applications require layers of aluminum alloys, such as Al -- 2% Si and Al -- 4% Cu -- 2% Si. Because different elements have different vapor pressures, alloys can be difficult to deposit by evaporation techniques. In contrast, deposition of alloy layers by sputtering is relatively easy.
Many previous attempts to deposit thick aluminum layers by sputtering were unsatisfactory or uneconomical because the deposition rates were too low. Advances in magnetically enhanced sputtering apparatus now provide much higher deposition rates. In particular, sputtering sources of the type disclosed by John S. Chapin in "The Planar Magnetron", Research/Development, Vol. 25, No. 1 (January 1974) enable deposition rates which are comparable to those typically obtained by electron-beam heated evaporation equipment. Magnetron sputtering sources are also advantageous in that they confine the glow discharge plasma to a closed loop adjacent target, and thus reduce electron bombardment and other heating of the substrate.